1. Field of the Invention
The present invention relates generally to frequency synthesizers.
2. Description of the Related Art
The term wireless in modern wireless communication systems refers to the transfer of information over a distance without the use of electrical conductors. Wireless systems facilitate communications that would be impossible or impractical to implement with the use of wires. Examples of these wireless access systems include:                Wideband Code Division Multiple Access (WCDMA) which utilizes direct sequence code division multiple access (CDMA) techniques;        Time Division—Synchronous Code Division Multiple Access (TD-SCDMA) which uses a combination of synchronous CDMA and Time Division Duplex TDD techniques; and        Worldwide interoperability for Microwave Access (WiMAX) which uses orthogonal frequency division multiplexing (OFDM) and is particularly structured to provide “last mile” broadband and internet access as an alternative to conventional systems such as cable and Digital Subscriber Line (DSL).        
Many wireless communication systems (e.g., mobile networks) include a series of base stations which are widely distributed through the network. In order to reduce radio-frequency (RF) losses, these base stations are often configured with the RF portions of transceivers (often referred to as remote radio units (RRU's)) positioned close to associated antennas and with the baseband portions of the transceivers positioned at a central location (the hub or base band unit (BBU)) of the base station along with other electronics that conducts various hub processes (e.g., baseband signal generation, modulation, demodulation, coding and framing). The hub typically interfaces, in turn, with a core network which handles other network functions (e.g., processing of subscriber calls).
Baseband signals between the RRU's and the hub are generally carried through an optical link which comprises optical fibers and associated serializer/deserializer (SerDes) circuits that convert signals to serial format for passage through the optical fibers. One of these baseband signals is a network clock that is generated by a network master controller in the hub. The network clock is sent through the optical link to each of the RRU's where it is used to generate and synchronize local oscillator signals and sampling clock signals for up-conversion, and down-conversion, for analog-to-digital and digital-to-analog conversion, and for baseband processing.
Although this base station structure has the advantage that it reduces RF losses between antennas and associated transceivers, it presents other network problems that may cause excessive clock jitter. For example, excessive length of the optical link is often sufficient to induce substantial jitter in the network clock and the subsequent use of clock recovery algorithms may also be the source of excessive jitter. In any case, if the resultant jitter is not significantly reduced during generation of local oscillator signals and other transceiver clock signals, the quality of received and transmitted signals will be degraded.